The present invention relates to a method for measuring the nonlinearity of an analog front end (AFE) system, such as an analog/digital converter (ADC), and more in particular, to a method for measuring accurately the differential nonlinearity (DNL) and integral nonlinearity (INL) of an AFE system.
With respect to the conversion characteristic of an AFE system, e.g., an ADC, the DNL and INL both are very important specifications. Conventional methods usually utilize an accurate signal source to first measure the DNL of an AFE system. Afterwards, the INL of the AFE system is obtained by accumulating the DNL of the AFE system. Related prior arts refer to the following:
1. U.S. Pat. No. 4,352,160;
2. U.S. Pat. No. 5,712,633;
3. xe2x80x9cMixed-signal Testing Tutorial Class Notes,xe2x80x9d at European test conference, Rotterdam, on Apr. 19, 1993.
Hereinafter, the disadvantages of the conventional approaches will be described.
Note that an accurate signal source is necessary for the conventional approaches. When an inaccurate signal source is employed to measure the DNL and INL of an AFE system, it will introduce error into the DNL and INL. The DNL with little error induced by the inaccurate signal source may be permitted. However, the error of INL, which accumulates that of the DNL into a large value, can not be negligible. Therefore, only permissible DNL but no permissible INL can be obtained for an AFE system when an inaccurate signal source is employed to measure the nonlinearity of the AFE system.
Besides, shifted reference voltage existing in the signal source and/or poor-calibrated AFE system will induce D.C. drifts which makes the measured DNL diverge from accurate DNL. Furthermore, in conventional approaches, the computed INL of the AFE system by accumulating the inaccurate DNL will diverge from accurate INL more.
The foregoing and other state-of-the-art approaches for measuring the DNL and INL of an AFE system indicate the need for a new method of providing accurate DNL and INL measurements of the AFE system that can be implemented without the requisite of well-calibrated equipment and a high-accuracy signal source. It is also desirable that the measurements be immune to D.C. drifts induced by the signal source and/or AFE system. The present invention is directed toward satisfying the aforesaid need. In addition, the invention is to provide a method for measuring the INL of an AFE system by transferring from the output data of the AFE system rather than by accumulating the DNL. Further, the DNL of the AFE system is obtained by differentiating the INL. Therefore, the INL and DNL of the measured AFE system according to the invention both are only slightly affected by the accuracy of the inputted signal source slightly.
It is an objective of the invention to provide an improved method of measuring the DNL and INL of an AFE system, e.g., an analog/digital converter.
It is another objective of the invention to provide a method of measuring the DNL and INL of an AFE system that does not require well-calibrated equipment and a high-accuracy signal source.
It is another objective of the invention to provide a method of measuring the DNL and INL of an AFE system whereby the measurements are immune to D.C. drifts induced by the signal source and AFE system.
It is another objective of the invention to provide a method of measuring the DNL and INL of an AFE system whereby the measurements are affected slightly by the accuracy of inputted signal.
According to the invention, a method is provided for measuring an integral nonlinearity (INL) data and a differential nonlinearity (DNL) data of an analog front end (AFE) system corresponding to a predetermined ideal code. In a conversion of a signal by the AFE system, the signal is converted by the AFE system into N successive true codes at N corresponding successive coded steps. N successive ideal codes, based on to an ideal conversion of the signal by the AFE system, are computed at the N corresponding successive coded steps. Each of the N true codes includes a D.C. drift component contributed by a D.C. drift in the conversion, a signal component substantially contributed by the signal, a noise component contributed by a noise in the conversion and a harmonic distortion (HD) of the signal in the conversion. The N ideal codes includes the predetermined ideal code. The method is first to extract the noise component and HD component of each of the N true codes to obtain N error codes at the N coded steps. Afterwards, the method is to map each of the N error codes by one corresponding ideal code in accordance with the N coded steps. The method is then to average the error codes mapped by the predetermined ideal code to obtain the INL data. The method is then to differentiate the INL data with respect to the predetermined ideal code to obtain the DNL data. In the method, the noise and THD components, actually contributing the nonlinearity of the AFE system, are extracted, and then derived into the INL data. Afterwards, the DNL data is obtained by differentiating the INL data. Therefore, the method is implemented without the requisite of well-calibrated equipment. Moreover, the INL data and DNL data both are only slightly affected by the accuracy of the inputted signal:
The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings.